In a conventional engine starter, the output shaft axially slidably carrying a pinion which meshes with a ring gear, and the solenoid device for axially driving the pinion were typically arranged in a parallel relationship. According to such a double-shaft type engine starter, because the solenoid device projects sideways from the electric motor, the radial dimension of the starter was inevitably significant, and a substantial restriction was imposed on the necessary mounting space.
To overcome such an inconvenience, there have been proposed a number of coaxial engine starters which have a solenoid device disposed around the output shaft. The Applicant (Assignee) of this application previously proposed in Japanese laid open patent publication No. 8-319926 (United States patent application No. 08/653,873 filed May 28, 1996) a coaxial engine starter that can reduce the axial dimension without complicating the overall structure.
According to this prior patent application, the pinion is coupled with the output shaft via a one-way clutch and a helical spline so that the pinion is pushed toward the ring gear as the output shaft turns. The pinion is further thrust forward by a moveable core or an armature which is magnetically actuated so as to more positively mesh with the ring gear, and the axial dimension is reduced by forming the armature from armature inner and outer members which are nested one in the other. However, if the thrust force is insufficient when the pinion is about to abut the side surface of the ring gear, the pinion may fail to mesh with the ring gear. Therefore, it is desired to increase the magnetic attractive force acting on the armature, and the thrust force of the pinion.
Furthermore, as it is desired to more reliably push the pinion into engagement with the ring gear, a coil spring is interposed between the armature inner member and a fixed part consisting of a top plate for providing a spring force for pushing out the pinion in addition to that provided by the magnetic attractive force.
However, this arrangement has the problem that the coil spring extends as the armature inner member moves in the direction to push out the pinion, and the spring force of the coil spring diminishes by a corresponding degree. To overcome this problem, it is conceivable to move the coil spring with the movement of the armature outer member. By so doing, it is possible to reduce the extension of the coil spring by the amount of the movement of the armature outer member.
According to such an arrangement for supporting the coil spring from the side of the armature outer member, it is conceivable to integrally provide an inwardly directed radial flange on the inner periphery of the armature outer member to engage one of the coil ends of the coil spring. However, it would be difficult and costly to carry it out. Furthermore, the magnetic flux would leak from the flange which is made of the same magnetic material as the armature outer member, thereby reducing the magnetic attraction on the magnetic outer member, and, also, the armature inner member would be magnetically attracted toward the flange (or in the direction to pull it back). The inwardly directed radial flange may also create the problem that the corresponding coil end of the coil spring may shift sideways, and interfere with the armature inner member or other components.